Cathode ray tubes (CRTs) are the most widely used electrooptic image transducers ever devised. Their major usage began with the introduction of television news and entertainment broadcasting and has now spread into all systems of visual image presentation, information displays and computer data readout. Such displays are generated by rapid movement of an information modulated electron beam by means of magnetic fields introduced by coils mounted externally on the neck of the tube. An accelerating voltage of several thousand volts assures projection of the electron beam onto the phosphor coated screen with sufficient energy to cause it to write with a brilliant glow.
When such a beam is deflected according to an orderly procedure of movement (such a from left to right across the screen starting at the upper left and proceeding downward), a rectangular patch is recorded for the viewer's visual retention. When repeated often enough, the illusion of continued presence of such an image field is created. Variations in the intensity of the electron beam as it moves repeatedly through such a scanned raster register in the eye and mind of the viewer as recognizable geometric images.
It is common practice to move the electron beam across the screen at a uniform velocity both in the direction of the rapid line scan (generally horizontal) as well as in the slower (generally vertical) field scanning direction. Linearly variant current ramps are applied to the deflecting coils on the neck of the tube to accomplish this. Each current ramp terminates as the electron beam reaches its extreme position and then reverses direction quickly causing the beam to return to its starting point. In order to support the illusion of continued image presence without tell-tale flicker becoming apparent, it is necessary to refresh the entire image at least fifty or sixty times per second. When a fine structure of many hundred scanning lines are to be included in the display so as to provide good image resolution, the line scanning rate may need to be as high as one hundred thousand sweeps per second. It is standard practice to use only about sixteen thousand for broadcast television purposes. For computer driven monitors and displays (e.g., VGA, Super VGA, High Resolution, etc.), the scanning rate is even faster.
The cathode ray beam accelerating voltage (i.e., second anode voltage) often ranges from twelve to thirty kilovolts, depending upon screen size and the desired brightness level of the image generated. The current requirement imposed upon this high voltage source may range from fifty or one hundred microamps for monochrome screen to one or two milliamperes for color cathode ray tubes. The high voltage source itself can be any available type of power generator provided it can support the current demand of the CRT and has an upper current limit of a few milliamperes beyond which the voltage collapses so as to protect servicing personnel from instant electrocution should they make accidental contact therewith.
Early in the development of television broadcasting, a unique type of second anode voltage supply for magnetically deflected cathode ray tubes was developed. This very simple and inexpensive high voltage power supply system is a supplement to the magnetic beam deflection technique previously described and makes use of the rapid current reversal rapidly and repeatedly returns the electron beam to its start of sweep position. However when an electric current changes value rapidly while flowing in an inductor such as a deflecting coil, it creates a substantial voltage across the terminals of the coil by self induction. Thus, a brief voltage pulse, reaching several hundred volts in magnitude, is generated on the terminals of the deflection coil as it caused "flyback" of the CRT beam, line-by-line.
As a source of voltage, this "flyback" pulse found on the deflection coil terminals is powerful and can stand heavy loading due to the low internal impedance of the deflection coil system. It is only necessary therefore to "step up" this voltage by means of a transformer. The several hundred volt level of the deflection coil can thus be raised to the several thousand volt (rectified DC) level required to supply second anode voltage for the tube.
In this manner a low cost combination line deflection and high voltage generator has been developed for use with cathode ray tubes. The flyback high voltage system has reached almost universal acceptance in all forms of commercial cathode ray display devices in spite of one important limitation, a visible geometric expansion of the rawer area as screen brightness is increased. This defect is due to falling second anode voltage or poor high voltage regulation. Poor regulation (e.g., about 10 to 15 percent or less), although acceptable for an entertainment television display, is unacceptable for such critical display systems such as ultrasonic medical imagery, word processing business machines, computer aided design screens, and other systems which are sensitive to exactness of image size.
Poor voltage regulation in the flyback-type high voltage system just described is due to the inherently high internal source impedance of the step-up transformer and pulse rectifying arrangement. The system operates in a completely open loop manner, and generally lacks the benefit of any current preload. Many prior art attempts have been made to stabilize the voltage of such power supplies:
1. Using a high voltage shunt transistor (e.g., a Zener diode) to load the supply until an external load is applied, thereby holding a fixed threshold voltage level. This approach wastes power and heats up the thyristor which leads to early destruction. PA0 2. Using a vacuum tube shunt regulator with or without voltage feedback loop control. This approach is also a power waster because the tube heater must also be activated; however, the components exhibit longer life. PA0 3. Using an adjustable flyback interval under feedback loop control, having either inductive or capacitive tuning means, to control flyback voltage. Such a technique requires complex adjustment and the variable tuning is generally visible on the screen. PA0 4. Using a separate flyback high voltage supply section independent of the deflection yoke section with an adjustable DC voltage source under feedback loop control. Although this is successful and is widely used, it is expensive and power inefficient due to near duplication of components. PA0 5. Using a supplementary adjustable DC source added to the high voltage winding to offset internal drop as it occurs. This also requires a large number of additional components and power is inefficient. PA0 6. Using a pulse width modulated feedback loop which supplies sufficient additional voltage, through an energy storage transformer, during the generation of each flyback pulse in order to maintain the high voltage output of the flyback transformer near a regulated level (e.g., U.S. Pat. No. 4,614,899 to Webb et. al.) Although voltage regulation is satisfactory, deflection yoke current is affected and image size is not exact.
Finally, while the above problem is characteristic of flyback-type high voltage power supplies, it is symptomatic of all pulsed voltage power supplies.
Thus, while there has been considerable progress in the design of voltage regulators for a flyback type high voltage power supply for a CRT, there are still some important shortcomings. In particular, while television displays operate under one standard frequency scanning rate, modern video monitors must be capable of operating in accordance with a plurality of video driving protocols (e.g., VGA etc.) In addition, even with voltage regulation, it is unacceptable for video displays to be affected by changes in current flow through its deflection yoke. An improved voltage regulator for a flyback-type voltage supply would be welcomed by users of video monitors, particularly those who demand exactness in image size.